The Pactborn are the kin of warlocks, born already doomed by a pact they never swore to an otherworldly power they may not even know. Whether a tiefling or other subrace, all Pactborn come into the world with an appearance and temperament marked by the otherworldly patron of their warlock parent. Although the Pactborn are quite diverse as a race and blanket statements rarely apply, lives of isolation and wandering are common for them. Often rejected by their parents out of shame and wider society out of fear, the Pactborn seek out the company of those that will accept them as they are or turn bitter and use their appearance to manipulate and intimidate others. Not all children of warlocks are Pactborn but what separates those that are and those that aren’t is not understood.
The Pactborn were created because I felt like something more interesting could be done with tieflings than what was presented in the PHB. I liked the idea of the tieflings fitting into a wider category of people more than the idea of making a broad selection of subraces that fit into an overarching tiefling race so I found what I thought was an obvious point of common ground for tieflings to have with others (otherworldly influence) and went from there!
Edit: In the original blog post the link above took you straight to the Pactborn pdf. As for July 18th, 2016 the link has been changed to the Pactborn on the DM’s Guild!